In electric wires used in movable equipment such as automobiles and airplanes and industrial equipment such as robots, an insulating layer is removed from an end portion to expose a conductor, and a terminal fitting is attached to the exposed portion. The terminal fitting may be of a variety of forms. For example, when the terminal fittings are connected to each other, a female terminal fitting 100F provided with a female fitting portion 130 or a male terminal fitting 100M provided with a male fitting portion 140, such as shown in FIG. 1, is used as an electric connecting portion that electrically connects the two terminal fittings.
The female terminal fitting 100F and the male terminal fitting 100M shown in FIG. 1 are both of a crimping type provided with a wire barrel portion 110, which has a pair of crimping pieces as main components, as a conductor connecting portion for connection to a conductor 210 provided at an electric wire 200. As shown in FIG. 1A, in the female terminal fitting 100F, a tubular female fitting portion 130 is provided to extend from one side of the wire barrel portion 110, and elastic pieces 131, 132 disposed opposite each other are provided inside the tubular body. In the male terminal fitting 100M, a rod-shaped male fitting portion 140 is provided to extend from one side of the wire barrel portion 110. Where the rod-shaped male fitting portion 140 is inserted into the tubular body of the female fitting portion 130 as shown in FIG. 1B, the male fitting portion 140 is strongly grasped by the biasing force of the elastic pieces 131, 132, and the two terminal fittings 100F, 100M are electrically connected to each other. In FIG. 1, only the female fitting portion 130 is shown by a sectional view to facilitate the understanding.
Copper materials such as copper or copper alloys, which excel in electric conductivity, are mainly used as constituent materials for conductive bodies or terminal fittings of electric wires. In recent years, the possibility of using, as constituent materials, aluminum or aluminum alloys (referred to hereinbelow as Al alloys), which have a specific gravity of about ⅓ that of Cu, in order to reduce the electric wires in weight has been studied (Japanese Patent Application Publication No. 2010-272414).
Japanese Patent Application Publication No. 2010-272414 suggests to provide a plated layer on the surface of the above-described fitting portion in order to reduce the electric connection resistance when the terminal fittings are connected to each other. The plated layer includes a Zn layer, a Cu layer, and a Sn layer, or a Zn layer, a Ni layer, a Cu layer, and a Sn layer, in the order of description from the base material. Since Sn (tin) is soft and easy to deform, sufficient conduction between the terminals fittings that are to be connected can be ensured by Sn deformation. In other words, by causing a Sn layer to function as a contact material, it is possible to reduce the connection resistance. Further, by covering the base material surface with such a plate layer, it is possible to prevent the oxidation of the Al alloy constituting the base material.
When a Sn layer is provided on the outer circumference of a terminal fitting constituted by an aluminum alloy, it is desirable that the Sn layer be closely attached to the terminal fitting over a long period of time. In particular, when the Sn layer is used as a contact material, it is desirable that the Sn layer have high peel resistance, since the peeling of the Sn layer increases the connection resistance.
The results of the investigation conducted by the inventors demonstrate that where a Zn layer is provided as an underlayer, as described in Patent Document 1, the Zn layer is eluted with time because of contact corrosion of dissimilar metals and, therefore, the Sn layer provided on the outer circumference of the Zn layer can peel off. For this reason, it is desirable to develop an aluminum-based terminal fitting in which a Sn layer can be sufficiently present, without falling off, over a long period of time.